Separator technology is commonly used in wells worldwide, be it a test separator or a production separator. The separators used in sand control completions, for example, are particularly prone to filling with sand. This issue is particularly problematic in relation to off-shore installations where the shut-down of production to clear out a separator can cause significantly delayed production and related financial losses.
Sand separators are also often to protect test or production equipment when large amounts of sand are anticipated as part of the process at hand (hydraulic fracturing, sand control applications, or oil sand applications for example). Prior warning of sand accumulation in a separator is key to minimizing downtime and delayed production, which is typically accomplished via planning and scheduling of routine separator maintenance.
As an example, exploration for methane, or natural gas, involves injection of high-pressure fluids (mostly water with sand) directly into underground rock formations expected to yield natural gas. In hydraulic fracturing procedures, water pressure fractures the rock strata, whereupon entrapped natural gas escapes into a well bore and is captured at the surface. Hydraulic fracturing fluid is recovered from the exploration wells and disposed of, usually by hauling it off in trucks to a remote disposal site. This fluid contains a considerable amount of fracturing sand. The sand is used to help hold open cracks to maximize escape of natural gas from within the strata. Fracturing sand is also used to clean and etch formations so to promote maximum gas delivery. The sand present in fracturing fluid doesn't all remain lodged in the formation, so some returns to the surface in what is called the “flowback” from the well. The flowback fluid includes a significant quantity of injected fracturing sand, as well as silt and rock debris flushed from the rock strata. Such sand and debris can clog or damage pipes, valves, pumps, and other portions of the system. Sand separators prevent these particulates from clogging and damaging the system, but only to the extent that the sand separator is functional. This is merely provided as an example to illustrate one use for sand separators.
In general, a sand separator is used to separate sand or other solids from a liquid/solid mix, and for continued operation of sand separators, a reliable indication of the level of sand in the separator is required. If the sand level is not correctly calculated, there is a risk the sand separator will over-fill. Once over-filled, the typical remedy is to halt the process at hand and manually empty the sand and debris from the separator. Of course, during such corrective actions neither the sand separator nor the production equipment attached thereto is usable, thus facilities incur production down-time and related financial losses.
There is a need for means to eliminate or reduce sand separator clogging. The embodiments described below overcome these and other problems and an advance in the art is achieved. The embodiments described below provide a sand separator that detects the sand level in the collection chamber having a vibratory meter.
Vibratory meters, such as vibratory densitometers and vibratory viscometers, typically operate by detecting motion of a vibrating element that vibrates in the presence of a fluid material to be measured. Properties associated with the fluid material, such as density, viscosity, temperature, and the like, can be determined by processing measurement signals received from motion transducers associated with the vibrating element. The vibration modes of the vibrating element system generally are affected by the combined mass, stiffness, and damping characteristics of the vibrating element and the surrounding fluid material.
One example of a vibratory density or viscosity meter operates on the vibrating element principle, wherein the element is a slender tuning fork structure which is immersed in the liquid being measured. A conventional tuning fork consists of two tines, typically of flat or circular cross section, that are attached to a cross beam, which is further attached to a mounting structure. The tuning fork is excited into oscillation by a driver, such as a piezo-electric crystal for example, which is internally secured at the root of the first tine. The frequency of oscillation is detected by a second piezo-electric crystal secured at the root of the second tine. The transducer sensor may be driven at its first natural resonant frequency, as modified by the surrounding fluid, by an amplifier circuit located with the meter electronics.
When the fork is immersed in a fluid and excited at its resonant frequency, the fork will move fluid via the motion of its tines. The resonant frequency of the vibration is strongly affected by the density of the fluid these surfaces push against whilst the fluid viscosity has a significant effect on the bandwidth. As the viscosity of the fluid changes, the overall damping forces change, changing the bandwidth and with it the “Q” or quality factor of the sensor. An electronic circuit may excite the sensor into oscillation alternately at two positions on a frequency response curve, and in doing this, the quality factor (Q) of the resonator may be determined as well as the resonant frequency. By measuring certain periods related to the frequency response curve, the viscosity of a fluid can be calculated.
In particular, the viscosity of a fluid can be measured by generating vibration responses at frequencies ω1 and ω2 that are above and below a resonant frequency ω0 of the combined fluid and vibratory sensor. At the resonance frequency ω0, the phase difference Φ0 may be about 90 degrees. The two frequency points ω1 and ω2 are defined as the drive frequencies where the drive signal phase and the vibration signal phase differ by the phase differences Φ1 and Φ2, respectively. The phase difference Φ1 may be defined as the point where the phase difference between the drive signal phase and the vibration signal phase is about 135 degrees, for example. The phase difference Φ2 may be defined as the point where the phase difference between the drive signal phase and the vibration signal phase is about 45 degrees, for example.
The distance between these two frequency points ω1 and ω2 (i.e., the difference in frequency between ω1 and ω2) is used to determine the term Q, which is proportional to viscosity and can be approximated by the formula:viscosity≈Q=ω0/(ω1−ω2)
The resonant frequency ω0 is centered between the two frequency points ω1 and ω2. Therefore, the resonant frequency ω0 can be defined as:ω0≈0.5*(ω1+ω2)
The frequency points, ω1 and ω2, are determined during operation when the sensor element interacts with the fluid to be characterized. In order to properly determine the frequency points ω1 and ω2, the drive system uses a closed loop drive, driving the sensor element to alternate between the two phase difference points Φ1 and Φ2) and recording the vibration frequencies ω1 and ω2 at these points. By using a closed-loop drive, the prior art drive system ensures that the phase difference measurement is stable when the vibration frequencies ω1 and ω2 are determined. This serves as an example of how phase may be used to calculate viscosity by meter electronics.
By orienting a vibratory meter in a sand collection reservoir of a sand separator, and measuring changes in pickoff sensor signal strength and/or signal phase differences, the liquid/solid interface level in a sand separator is rendered detectable, as is disclosed herein.